It’s a perfect day for flying—clear skies, no wind—and you’re taking a buddy for his first flight in a small airplane. You can see the excitement in his eyes and it reminds you that this is why you love to fly. After a quick preflight run-up you’ll be ready to go. At 1800 RPM you select the left magneto, And…Whoa! A 200 RPM drop! Now what? The POH says, “RPM drop should not exceed 150 RPM on either magneto.” So, what’s the big deal? The magneto is working. What harm can come from a quick local flight?

Without a lengthy technical explanation of magneto theory, let’s take a look at the possible causes of a failed magneto check. First, though, let’s talk about a “good” mag check. That’s the one spelled out in the POH. For a 172S it states, “RPM drop should not exceed 150 RPM on either magneto or 50 RPM differential between magnetos.

For a successful mag check:

1. Both magnetos must be timed correctly.Most pilots are awarethat the magneto must be adjustedso that it produces the spark at thecorrect time in relation to engine rotation.But, did you know there’s asecond part to timing a magneto?This is the internal timing of thepoints, i.e., the points must open atthe correct time in relation to the rotatingMagnet.

2. The spark plugs must be clean and functioning correctly.If oneor more of the plugs are full of leador oil, they will not fire efficiently.Also, and this is very important, ifthe plug has ever been dropped,there’s a good chance the ceramicinsulator is cracked. This will causeit to short to ground instead of firingacross the post.

3. The sparkplug wiring harness must be functioning correctly.It shouldn’t be chaffing on theengine or baffling. Also, thereshouldn’t be any tight bendsthat may break the internalconductor.

4. The P-lead must be in place and not chaffed or shorted.This is thesmall wire that goes fromthe magneto back to theignition switch. The switchturns the magneto off bygrounding the p-lead.

Now, let’s take a look at somefailed mag check scenarios and theirpossible causes.

Diagnosis:This is most often caused by a dirty or lead-fouled sparkplug. Usually, you can get it to clear up by running the engine up to a cruise RPM setting (something greater than 2000) and leaning the engine. Important! Do this while running on both magnetos. I’ve had many pilots say they couldn’t clear the fouled plug using this procedure, but, as it turns out, they did it while running on only the bad magneto. Remember, you’re trying to generate enough heat in the cylinder to clean out the bad plug, so you need to have both magnetos on. It will generally clear the sparkplug in about 30-60 seconds. Also, keep in mind that you may need to have you’re A&P clean your plugs soon – they’re most likely getting full of lead.

Scenario #2: One magneto has excessive RPM drop, but the engine continues to run smoothly.

Diagnosis:You can try to clear the plugs like we did in scenario #1 and you may get lucky, but the most likely culprit is an incorrectly timed magneto. If the internal points are timed incorrectly, you will simply get a very weak and ineffective spark. If the magneto is timed incorrectly to the engine, this can cause serious damage to the engine. This needs to be addressed by a certified technician before flying.

Scenario #3: No RPM drop on one magneto

Diagnosis:Could be a timing issue, but the most likely cause is a broken P-lead. When you place the ignition switch in the “R” position, it grounds the left magneto’s p-lead. But, if the plead is broken, obviously it’s not going to ground the magneto and therefore not turn it off. You can confirm this by reducing the engine RPM to idle and briefly turning the ignition switch to “off.” (This should only be done at low idle!) The engine should start to die, then quickly turn the switch back to “both.” If the engine doesn’t start to quit and continues to run in the “off” position, there’s a broken p-lead. Keep in mind, with this fault the magneto is always “HOT” and a very small movement of the propeller can start the engine. This is a very dangerous situation and a certified technician should address it immediately.

Scenario #4: Engine quits running when one magneto is selected

Diagnosis: Well, this one’s obvious and there shouldn’t be any temptation to go fly. You either have a dead mag or a shorted p-lead.

Please note: when you select the dead magneto, you turn off all spark to the engine. However, fuel continues to be pumped into the engine and it’s not being burned. That unburned fuel then passes out of the cylinder into the exhaust system. If you move the switch back to “both,” you will ignite all that fuel and may hear a loud BOOM. Now you’ll not only need to have your magneto fixed, but you may be buying a new muffler (if not a complete exhaust system) as well. The moral of this story—if you’re running at 1800 RPM and select a dead magneto, don’t turn the key back. Instead, just let the engine quit, wait a few moments, and then start the engine normally. You can then taxi back to your maintenance shop.

The magneto issues mentioned above are the most common, but there are many others that can arise from ignition system problems. So, back to the situation in the opening paragraph, is it ok to go ahead with that local flight? The answer is a resounding “NO!” The parameters in the POH are there to protect you and your engine. Sure, the plane may fly, but you may not be doing your engine any favors. Get your maintenance shop to check it out.